Enhanced approach to match damage-equivalent loads in rotor blade fatigue testing

Abstract. In the design process of current wind turbine blades a critical step is the certification testing to confirm design assumptions and requirements. To demonstrate reliability in fatigue testing the blade shall be loaded in all areas of interest to the load levels, which at the end of such test campaign adequately represent the blade lifetime. These loads are typically derived from aero-elastic load calculations with a combination of different design load cases in the form of accumulated bending moment distributions. The current practice includes two fatigue test sequences, which are aligned with first flapwise and lead-lag modes respectively with the aim to reach defined target bending moment distributions. These two test sequences combined may not cover all areas of interests and some areas could be tested insufficiently. Also in some areas the conventional target bending moment formulation does not represent fatigue damage of the material correctly, as it is not derived from a stresses or strain based damage calculation and does not allow for mean load correction. The aim of this work is to demonstrate these shortcomings on a particular test case and propose an enhanced method to derive representative target loads, which cover all areas of interest and are strain proportional, allowing for correct material damage accumulation and mean load correction. It is shown for the test case that compared to the conventional methods the enhanced target loads require 16 % higher test loads at certain positions along the blade within the four main load directions and even more for load directions in between.